Mobile communication techniques have become a widely used communication manner for their advantages such as flexibility and convenience since 1980s. In a lot of mobile communication standards, CDMA cellular communication techniques show great potential for their features associated with large capacity, simple frequency planning, good communication quality and small electromagnetic interference. IS-95 CDMA cellular communication system proposed by Qualcomm Inc. and rapidly developed all over the world uses this CDMA cellular communication technique. Several candidate schemes of the third generation of digital cellular communication system are established on the basis of CDMA techniques.
In mobile communication systems, the stability of initial frequency in a mobile terminal is limited to about 1 ppm because of the restrict by volume and cast, etc. This results in approximate several hundred Hz to several KHz frequency difference between a base station and a mobile terminal. Therefore, it is necessary to introduce an automatic frequency correction (AFC) function into mobile terminals, thereby to decrease the degradation of the system performance.
Multipath fading which causes serious multipath interference exists in a mobile communication system. In designing an AFC, the adverse effects caused by the multipath fading channel should be considered. In a CDMA mobile cellular communication system employing a spread spectrum scheme, in general, it is necessary to transmit pilot signals with confirmation information. The amplitude and phase information of multipath signals may be evaluated by receiving the pilot signals. With respect to the fixed frequency-offset, which is overlapped on the multipath fading signals, in both transmitting side and receiving side, it is possible to extract the frequency-offset evaluating values of both transmitting side and receiving side by means of simple processing. A desired AFC function may be obtained by suitably adjusting the local frequency reference source of a mobile terminal by using the evaluating values. The method of extracting the frequency-offset information and whereby adjusting the local frequency reference source is referred to as AFC loop design.
The AFC loop design of a CDMA receiver need to meet the requirements of fast response speed, returning to active state at time when a mobile terminal turns on or loses lock, and compensating frequency difference between transmitting side and receiving side in a short time in addition to operate stably in the environment of multipath fading channel.